BIOGENIC NONMETHANE HYDROCARBON EMISSIONS ESTIMATED FROM TETHERED BALLOON OBSERVATIONS

A new technique for estimating surface fluxes of trace gases, the mixe d-layer gradient technique, is used to calculate isoprene and terpene emissions from forests. The technique is applied to tethered balloon m easurements made over the Amazon forest and a pine-oak forest in Alaba ma at altitudes up to 300 m. The observations were made during the dry season Amazon Boundary Layer Experiment (ABLE 2A) and the Rural Oxida nts in the Southern Environment 1990 experiment (ROSE I). Results from large eddy simulations of scalar transport in the clear convective bo undary layer are used to infer fluxes from the balloon profiles. Profi les from the Amazon give a mean daytime emission of 3630 +/- 1400 mu g isoprene m(-2) h(-1), where the uncertainty represents the standard d eviation of the mean of eight flux estimates. Twenty profiles from Ala bama give emissions of 4470 +/- 3300 mu g isoprene m(-2) h(-1), 1740 /- 1060 mu g alpha-pinene m(-2) h(-1), and 790 +/- 560 mu g beta-pinen e m(-2) h(-1), respectively. These results are in agreement with emiss ions derived from chemical budgets. The emissions may be overestimated because of uncertainty about how to incorporate the effects of the ca nopy on the mixed-layer gradients. The large variability in these emis sion estimates is probably due to the relatively short sampling times of the balloon profiles, though spatially heterogeneous emissions may also play a role. Fluxes derived using this technique are representati ve of an upwind footprint of several kilometers and are independent of hydrocarbon oxidation rate and mean advection.